1. Field of the Invention
The present invention relates to wafer metrology, and more particularly to obtaining overlay measurements for a semiconductor wafer using zero-order cross polarization measurements obtained from a periodic grating formed on the wafer.
2. Related Art
Semiconductor devices/circuits are formed on semiconductor wafers by depositing and patterning layers of materials. In general, the features of the devices/circuits are formed onto the layers of deposited materials using a patterning process.
In a typical patterning process, the features of the devices/circuits are laid out, one layer at a time, on a series of photomasks (masks). The layout of the features of the devices/circuits on the masks are transferred, one mask at a time, onto the deposited layers of materials. Misalignment of these masks, which is generally referred to as xe2x80x9coverlay errorxe2x80x9d, can adversely affect the performance of the devices/circuits.
To reduce overlay error, alignment marks, such as box-in-box or frame-in-frame overlay marks, are typically patterned onto the wafer and on layers deposited onto the wafer. At present, optical imaging systems are widely used to detect these alignment marks. However, a conventional optical imaging system typically has an accuracy of only about 5 to 10 nm. The continual shrinkage in the feature sizes of devices/circuits will likely require greater accuracy.
In an exemplary embodiment, overlay measurements for a semiconductor wafer are obtained by forming a periodic grating on the wafer having a first set of ridges and a second set of ridges. The first and second sets of ridges are formed on the wafer using a first mask and a second mask, respectively. After forming the first and second sets of gratings, zero-order cross polarization measurements of a portion of the periodic grating are obtained. Any overlay error between the first and second masks used to form the first and second sets of gratings is determined based on the obtained zero-order cross polarization measurements.